Civil & Structural Engineering

Footing Size Calculator

Estimate isolated pad or strip footing size from service load, allowable soil bearing pressure, rounding, and footing thickness.

Formula Square side = sqrt(W / qallow)Reviewed Sep 8, 2026

A footing size calculator answers the first planning question for a shallow foundation: how much soil contact area is needed so the service load does not exceed the allowable soil bearing pressure? This tool sizes square, rectangular, circular, and continuous strip footings, rounds dimensions upward, checks the resulting soil pressure, estimates concrete volume, and flags a common projection-thickness issue. It is a preliminary bearing and quantity estimate, not a complete structural foundation design.

Calculation Bench

Footing Type

Load & Soil

01Column / Post Service Load

P · Unfactored vertical service load from analysis or a design note.

02Allowable Soil Bearing

qallow · Use a soil report when available. Presumptive residential soil is often 1,500-2,000 psf.

03Load Allowance

% · Extra load allowance for footing self-weight and supported fill; 10% is a common planning value.

Geometry & Rounding

04Quantity

N · Number of identical isolated footings.

05Footing Thickness

t · Used for concrete volume and projection warning; not a full shear/rebar design.

06Wall / Column Width

b · Optional. Used to check common footing projection rules.

07Round Up Increment

Δ · Dimensions are rounded upward to this increment before pressure is checked.

Solution

Enter a service load and allowable soil bearing pressure to size the footing footprint.

Square side = sqrt(W / qallow)

Formula Sheet

W = P \times \left(1 + \dfrac{a}{100}\right)

B = \sqrt{\dfrac{W}{q_{allow}}}

B = \sqrt{\dfrac{W}{q_{allow}r}},\quad L = rB

D = \sqrt{\dfrac{4W}{\pi q_{allow}}}

B = \dfrac{w\left(1 + a/100\right)}{q_{allow}}

q_{actual} = \dfrac{W}{A_{provided}}

PService Load
wLine Load
WAdjusted Load
qallowAllowable Soil Bearing
AreqRequired Area
AprovidedProvided Area
BWidth / Side
LLength
DDiameter
tThickness

Variables & Units

Symbol	Variable	Description	Common Units
P	Service Load	Unfactored vertical load carried by an isolated pad footing.	lbf, kN
w	Line Load	Service load per unit length carried by a continuous strip footing.	lb/ft, kN/m
W	Adjusted Load	Service load after adding the footing/backfill allowance.	lbf, kN
qallow	Allowable Soil Bearing	Allowable service-level soil pressure from a soil report or code table.	psf, ksf, kPa
Areq	Required Area	Minimum soil-contact area before rounding.	ft², m²
Aprovided	Provided Area	Actual rounded plan area used for the pressure check.	ft², m²
B	Width / Side	Footing width or square side length after rounding.	in, ft, m
L	Length	Rectangular footing length after rounding.	ft, m
D	Diameter	Circular footing diameter after rounding.	in, ft, m
t	Thickness	Footing thickness used for concrete volume and projection warnings.	in, mm

How to Use This Calculator

01Choose the footing type: Square Pad, Rectangular Pad, Circular Pad, or Strip / Wall.
02Enter the service load. Use column/post load for isolated pads or line load for strip footings. These should be service-level loads, not strength-factored design loads.
03Enter allowable soil bearing pressure from a geotechnical report when possible. If you only have a preliminary residential value, 1,500-2,000 psf is commonly used as a presumptive range, but local code and soil conditions control.
04Keep the default 10% load allowance unless you have a project-specific self-weight/backfill allowance. This accounts for the footing concrete and supported fill in a simple planning way.
05Enter thickness, quantity or run length, and the dimension rounding increment. The calculator rounds up before checking actual bearing pressure.
06Use local minimum footing dimensions, frost depth, and inspection requirements even when the bearing math suggests a smaller footing.
07Optionally enter wall or column width to get a projection warning. This does not replace code detailing, but it catches an easy-to-miss geometry issue.

How the Formula Works

For an isolated footing, the design service load is the entered column or post load multiplied by the allowance factor. Required bearing area is that adjusted load divided by allowable soil pressure.

For a square pad, the side length is the square root of required area. For a rectangular pad, the calculator uses the chosen length-to-width ratio, then rounds both dimensions upward. For a circular pad, diameter comes from the area of a circle.

For a continuous strip footing, the same bearing logic is applied per unit length: required width equals adjusted line load divided by allowable soil bearing pressure.

After rounding, actual bearing pressure is recalculated from the provided area. This is the value to compare with allowable bearing pressure; rounding upward should reduce actual pressure below the limit.

Concrete volume is calculated from provided plan area multiplied by thickness and quantity, or by strip width multiplied by run length and thickness.

Worked Example 01

Square pad for a 22,000 lb column load

Known

Service load: 22,000 lb
Load allowance: 10%
Allowable soil bearing: 2,000 psf
Rounding: nearest higher inch

Formula

Square side = sqrt(W / qallow)

Substitution

W = 22,000 × 1.10 = 24,200 lb; Areq = 24,200 / 2,000 = 12.1 ft²; side = sqrt(12.1)

Result

Use about 3.5 ft × 3.5 ft after rounding

The rounded footing provides slightly more than the minimum required area, so the actual soil pressure falls below the 2,000 psf limit.

Worked Example 02

Strip footing under a 1,200 lb/ft wall

Known

Wall line load: 1,200 lb/ft
Load allowance: 10%
Allowable soil bearing: 1,500 psf

Formula

Strip footing width = adjusted line load / qallow

Substitution

B = (1,200 × 1.10) / 1,500 = 0.88 ft

Result

Required width is about 10.6 in, so round up to a practical/code-compliant width

The bearing calculation may produce a narrow width, but residential codes often impose minimum widths and projection rules that can govern before bearing does.

Applications

01Preliminary isolated pad footing sizing for posts, columns, deck supports, porch supports, and small structural frames
02Quick strip footing width estimates for load-bearing foundation walls
03Checking whether a proposed footing footprint stays within allowable soil bearing pressure
04Estimating concrete volume for a preliminary footing quantity takeoff

Assumptions

01Loads are service-level vertical loads for preliminary bearing sizing, not strength-factored structural design loads.
02Bearing pressure is assumed uniform below the footing and the load is assumed to act at the centroid of the footing.
03The additional load allowance is a simplified percentage, not an iterative footing self-weight and soil-cover model.
04Soil bearing pressure should come from a geotechnical report or the governing building code for the actual site.

Where This Model Stops

01Does not design footing thickness, reinforcement, punching shear, one-way shear, bending steel, settlement, sliding, overturning, uplift, eccentricity, frost depth, drainage, or local-code detailing.
02Does not handle combined footings, strap footings, mat foundations, sloped footings, stepped footings, or eccentric column loads.
03Does not check minimum footing width/thickness tables, reinforcement detailing, embedment depth, frost protection, termite shields, or anchor requirements.
04Presumptive soil values are only hints. Expansive soil, fill, soft clay, groundwater, slopes, seismic conditions, and unusual loads require professional design.
05Construction footings are safety-critical and often inspected; final dimensions should be confirmed by a qualified engineer, building official, or approved project drawings.

References

[1]
2021 International Residential Code, Section R403 Footings
International Code Council
Prescriptive residential footing provisions, including footing widths and projection requirements.
[2]
Foundation Design Principles and Practices
Federal Highway Administration
Engineering reference for shallow foundation bearing, settlement, and geotechnical design context.

Frequently Asked Questions

Is this a complete footing design calculator?

No. It is a preliminary bearing-area and concrete-volume calculator. A complete footing design also checks thickness, reinforcement, one-way shear, punching shear, settlement, frost depth, drainage, eccentricity, and local code requirements.

What soil bearing value should I enter?

Use the allowable bearing pressure from a geotechnical report whenever possible. If you are only doing a rough residential check, many code tables use presumptive values such as 1,500-2,000 psf for ordinary soils, but the actual site may require a different value.

Why does the calculator add 10% load allowance?

The footing concrete and soil/backfill supported by the footing also load the ground. A 10% allowance is a common early estimate, but final design may calculate those loads directly.

How is this different from the Concrete Calculator?

The Concrete Calculator estimates volume from dimensions you already know. This Footing Size Calculator starts with load and soil bearing pressure to estimate the required footprint, then reports concrete volume as a secondary output.

Can I use this for deck posts?

It can help with a preliminary pad or pier-footing bearing check if you know the post load and soil bearing value. Final deck footing depth, diameter, frost protection, uplift, lateral loads, and local inspection requirements still need to be checked separately.

Why can code minimums govern before soil bearing?

Small loads can produce a mathematically small required area, but building codes often set minimum widths, thicknesses, depths, and projection limits for constructability, durability, and inspection consistency.

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